Anti-government forces in Syria have stepped up their offensive in the central part of the country, while taking control of much of Aleppo in the north.

Syrian forces struck back by carrying out air raids jointly with Russian forces.

The Syrian Observatory for Human Rights said some rebel forces launched a major offensive against regions controlled by the government of Syrian President Bashar al-Assad last Wednesday, capturing most of the key city of Aleppo.

They also seized settlements in the central province of Hama, a transportation hub that connects with the capital Damascus.

A Syrian state-run news agency and other media reported that Syrian and Russian warplanes jointly targeted militants in Aleppo and Hama on Sunday.

Against this backdrop, US National Security Adviser Jake Sullivan told CNN that, "We were not surprised that these rebels would try to take advantage of a new situation, in which the Syrian government's main backers, Iran, Russia and Hezbollah, were all distracted and weakened by conflicts."

The Syrian Observatory for Human Rights claimed the number of civilians and combatants killed since last Wednesday has increased to over 440.